Mempool.space never fails me.. ever.. I always check their estimate to get into the next block prior to sending any btc on chain. And they're always accurate, at least they have been for me

There's only two options, in the next block, or not.

No estimate can predict the arrival of new transactions or the fees they are paying, so all that granularity is brittle and unreliable.

I check mempool and if in a hurry (hardly ever) I pay a fee midpoint in the next block, still no guarantee, otherwise I pay 1sat/byte and wait.